Many electronic items emit radio waves, which can interfere with other sensitive electronic equipment. It is not always possible to completely eliminate this noise but only to reduce it to a tolerable level. The inventive suppresser assists in such noise reduction.
In one application, the invention is useful in attenuating emissions from wires connecting the flash tubes of high-voltage discharge systems, particularly series-injection-triggered strobe systems, which are used as strobe lights positioned to act as obstruction warning lights for aircraft safety. In that application, excessive electromagnetic interference is particularly worrisome because the lights are often used in proximity to airports, where the noise can interfere with navigational equipment.
Obstruction lights are also frequently located on towers. There are three basic types of systems attached to towers: (1) transmission-only antennas; (2) receive-only antennas; and (3) receive-and-retransmit systems. Systems in the last two categories are most susceptible to electromagnetic interference, because of the low signal levels involved. Cellular phone systems fall in into the third category. Because they work at approximately 800 MHz, cellular phone systems operate within the range of frequencies susceptible to interference.
An example of a strobe-type light beacon is shown in U.S. Pat. No. 5,155,666, which is commonly owned and which is hereby incorporated by reference. Referring to FIGS. 4 and 5 of the '666 Patent, the flash lamps (numerals 14, 16, and 18) are connected together in series. The connecting wires and the lamps together form a triangular shape. In such a lamp, experimentation has found that unwanted emissions occur primarily during the triggering period before the main discharge and cover a wide spectrum from essentially direct current to greater than 1 GHz. The emissions are proportional to the trigger voltage level and also depend on the flash tube characteristics. In addition, measurements have localized the source of most of the noise to the apexes of the triangle formed by the flash tubes and associated wires.
Voltages on the order of 15 to 20 Kv are required to reliably trigger the flash tubes and initiate the main discharge from the capacitor bank through the circuit. However, the noise is proportional to the trigger voltage. It is therefore necessary to attenuate the radiated emissions (noise) across a broad band of frequencies while maintaining a reliably high trigger pulse.
There have been various attempts to reduce emissions from strobe lights mounted on towers or at airports in close proximity to antennas. Most commonly, noise is masked by surrounding the flash head with a wire mesh "cage." Other encasement systems are also known but are generally not suitable for a light-emissive circuit. Other types of known circuits that have been designed in attempts to reduce noise include systems that use capacitors to attenuate high-frequency voltage in the discharge lines and grounding paths. Such systems are largely ineffective in reducing the electromagnetic emissions from the current on the wires across a broad spectrum.
It is also known to use a variety of shielding systems for wires and cables. For example, conventional ferrite cores that surround and shield wires are widely available. Such U.S. Pat. Nos. as 5,334,955; 5,355,109; 5,343,184; 5,287,074; 5,200,730; and 5,162,772 provide recent examples disclosing such ferrite cores, as well as fasteners configured to hold such cores in proximity to a wire or cable. However, ferrite cores have not been used to shield lighting fixtures, in part because light cannot pass through the cores or their housings and because it has not previously been recognized that the bulk of the noise is emitted from the wires between the flash tubes, as opposed to the tubes themselves.
It is a primary object of the invention, therefore, to provide a system for shielding wires from electromagnetic interference across a wide spectrum of frequencies.
It is another object of the invention to provide a system for attenuating emissions from high-voltage discharge systems and particularly from series-injection-triggered strobe systems.
It is another object of the invention to provide a system for blocking noise emitted from wires connecting the flash tubes of a strobe lamp.
It is another object of the invention to provide a system for substantially lowering electromagnetic emissions in strobe lights while not compromising the primary function of an obstruction warning light, to provide safety for aircraft.
It is another object of the invention to provide a system for blocking electromagnetic interference in an obstruction light while not blocking part of the light.
It is another object of the invention to provide a system for reducing the ability of an obstruction light to interfere with nearby navigational equipment, transmitters, or receivers.